Method and apparatus for radioactivity well logging



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METHOD AND APPARATUS FOR RADIOACTIVITY WELL LOGGING Filed March 22, 1945 a Sheets-Sheet s k u 5 g L x3 4 4 2 w \wgk w i u w k \v E W4 A s us i v m 4* l Patented Sept. 6, 1949 UNITED STATES PATENT OFFICE METHOD AND APPARATUS FOR RADIO- ACTIVITY WELL LOGGING Application March 22, 1945, Serial No. 584,184

Claims.

This invention relates to a method and an apparatus for logging wells and bore-holes and more particularly to a method for determining the nature and location of the various formations or strata traversed by a bore-hole by measuring variations in the natural or induced radioactivity of the formations.

This application is a continuation-in-part of my prior application, Serial No. 460,198, filed September 30, 1942, now U. S. Letters Patent No. 2,378,408, granted June 19, 1945.

The principal object of the invention is to provide a method and equipment by means of which a bore-hole can be logged with accuracy as great as or greater than is possible with the present radiological methods, and at a speed several times greater than that possible with present equipment. In case extreme accuracy should be required, this method can be used with a logging speed at least as high as that now possible with present methods and apparatus.

It is now well-known that the concentration of radioactive substances in the earth varies considcrably with changes in the stratography and a close correlation between changes in the strata and their radioactive content can be made. Shales are usually the most radioactive while sandstones and limestones are less active. Measurements of radioactivity along the bore-hole or well, therefore, provide a good method of locating the various strata and distinguishing between adjoining formations. A radioactive logging detector passed through the hole measures the intensity of the gamma rays emitted from the formations, these rays being spontaneously emitted from the decaying radioactive atoms. The penetration power of these rays is relatively great and the absorption by n" of steel casing is only about and it therefore is possible to obtain a good log through the casing in a well; moreover, the influence of the fluid in a well upon the measured intensity is practically negligible. If the annular space between the logging detector and the casing is filled with water or oil, the gamma ray intensity is reduced by only a small percentage. The radiological method therefore is able to produce accurate logs through the casing in wells filled with a fluid, where other logging methods are entirely unsatisfactory.

The speed with which a radioactive log can be taken is limited by the amount of detail or resolution one wishes to obtain. This resolution represents the thinnest stratum that can be identified. In the U. S. Letters Patent of myself and D. G. C. HaraNo. 2,397,078, granted March 19,

1946, and the patent of D. G. C. Hare, No. 2,397,071, granted March 19, 1946, forms of radiation detectors are disclosed which have proven to be many times more sensitive than ionization chambers and the conventional Geiger-Muller counters. The detector disclosed in Patent No. 2,397,073 comprises a plurality of elongated thin cathode plates disposed in slightly separated relation and a plurality of fine anode wires stretched between adjacent pairs of plates. The other detector disclosed in Patent No. 2,397,071 comprises a plurality of thin cathode discs disposed in parallel separated relation and arranged in a relatively long column. These discs are provided with several series of holes arranged in alignment and a thin anode wire is disposed so as to coincide with the longitudinal axis of each series. The cathode and anode of each detector is disposed in a suitable casing or envelope containing a gas at a comparatively low pressure and a source of potential of around 1000 volts is applied across the anode and cathode. A gamma ray striking one of the cathode plates may elect an electron into the gas, which electron is accelerated by the electric held from the oathode to the anode to such an extent that it ionizes the gas in the counter. New electrons and ions will thus be formed and these are accelerated by the electric field and in turn produce still additional new ions and electrons. The charge is collected on the anode and produces a comparatively high current pulse of the order of one microampere. These pulses can be amplified easily and fed through the logging cable to the surface.

The detectors described can be made of various lengths ranging anywhere from a few inches to several feet, the longer detectors being naturally more sensitive since they present a greater area to the action of the gamma rays while the shorter detectors, due to their higher resolving power, permit of the accurate logging of very thin formations. It has been demonstrated that with a high eiiiciency detector of the type described, of about three inches in diameter and five feet in length, a logging velocity of 3000 feet per hour results in a percentage error of 2.5%. When the speed is increased to 6000 feet er hour the error is 3.5%, and this shows that the error actually increases rather slowly with an increase in speed; however, while a detector five feet long will thus produce an accurate log when moving through the hole at a speed of 6000 feet per hour, it will not have a sufl-lclently high resolving power to determine accurately the location of strata which are much less than live feet in thickness. In

CROSS REFERENCE EXAMINEI 4 and of a length substantially equal to the thickness of the thinnest stratum to be logged.

A power pack or power converting means 3| is provided within the instrument to convert hole logging has been provided in which a pair of the current from the power source into the neceshigh emciency radiation detectors disposed in a sary voltages for operation of the instrumentalisingle housing suspended from a suitable cable ties within the housing 22. The high voltage for are adapted to be passed through the hole to be operation of the detectors 28 and 30 is impressed logged. The pulses from the detectors are conupon them from the power pack 3| through the ducted upwardly through the cable, being imconductors 32 and 33, respectively. The output pressed thereupon in such manner that, by suitsignals from the two detectors 28 and 30 are able devices at the surface, the pulses from either conducted to the amplifying channels 34 and 35 detector may be separately indicated or recorded, respectively. The signals or pulses from the deor if desired the total of the pulses from both detector 28 pass to amplifier 34 through the contectors may be indicated or recorded. In the ductor 62 and the conductor 66; and the output latter case, the effect secured is that of a single pulses from the detector are conducted to the detector of a length equal to the combined length amplifying channel through the conductor 62' of the two detectors, and because of this effect, and the conductor 66'. The necessary direct curthe instrument containing the two detectors may rent voltages for operating the amplifying tubes be passed through the hole ata high speed, say 20 in the amplifier channels 34 and 35 are supplied of 4000 to 5000 feet per hour or higher, and an accurate log will be obtained. When it is indicated that a thin formation is present, less in thickness than the combined length of the detectors, it may be logged at a slower speed, say of about 1000 feet per hour, while recording only the pulses from one of the detectors, preferably the shorter, and because of its higher resolving power, an accurate record is made of the nature and location of the thin formation.

For a better understanding of the invention reference may be had to the following description, illustrated by the accompanying drawings in which:

Fig. 1 is a vertical sectional elevation through a well or bore-hole showing a radiation detecting instrument in accordance with the present invention, suspended within the hole;

Fig. 2 is a diagrammatic sectional view of the instrument shown in Fig. 1, on somewhat enlarged scale;

Fig. 3 is a diagram of the detector and amplifying circuits used within the instrument; and

Fig. 4 is a diagram illustrating the selecting and amplifying circuits employed at the surface in accordance with the present invention.

Referring to the drawings, and more particularly to Figs. 1 and 2, a bore-hole or well It is shown as having been drilled through formations or strata such as [2, l4, l6, l8 and 20. For purposes of illustration it is understood that the formation I8 is fairly thin, say, about 15" in thickness and that each of the other formations l2, l4, l6 and 20 is at least three to five feet in thickness.

A housing 22, which may be of any suitable material and construction, preferably of metal, is adapted to be lowered and raised through the hole by means of a cable 24 passing over a measuring device or reel 26. Within the housing 22 are disposed a pair of elongated radiation detectors 28 and 30 sensitive to gamma radiation and preferably of the pulse-producing type such as is disclosed in the aforementioned patent of D. G. C. Hare No. 2,397,071. The detector 30 may suitably be, say 2 to 3 feet in length and the detector 28 about 15". These dimensions are by way of example and correspond to the dimensions of an instrument which has been used in actual service in accordance with the invention described herein. The detectors may be of other lengths, but it is preferred that the shorter be about onethird to one-half as long as the longer detector from the power pack 3| through the conductors I6, 82 and 85.

The cable 24 is preferably of the single-conductor, shielded type having a central conductor and a shield which serves as a return or ground for the various circuits involved. As shown in Fig. 2, in which the lower portion of the cable is shown on a somewhat larger scale, the central conductor in the cable 24 is designated by the numeral 25 and the shield, which is electrically connected with the shell or housing 22 of the instrument, is designated by the numeral 21. The instrument housing 22 thus may serve as a return or ground for the various circuits and constitutes a base point of common direct current potential for the various circuits and instrumentalities within the housing. The current for the operation of the various instrumentalities within the housing 22 is supplied through the cable from a suitable source at the surface, designated by the numeral 44. As illustrated, this may be a source of alternating current. Within the instrument, current for the operation of the various instrumentalities within the instrument housing 22 is taken off of the conductor 25 in the cable through the conductor 46 to the power pack 3|, the return being through the conductor 47 to the housing 22.

The amplifier channels 34 and 35 may be of any suitable type, for example, as disclosed in the U. S. Letters Patent of D. G. C. Hare N 0. 2,381,904, granted August 14, 1945, in which the pulses from the detectors 28 and 30 serve to modulate carrier current frequencies generated by oscillators within the amplifier, these modulated carriers then being conducted upwardly through the cable 24 to the equipment at the surface. However, it is preferred that the amplifier channels 34 and 35 be of a type yielding amplified and equalized pulses which are capable of bein differentiated by suitable equipment at the surface. Preferably, in accordance with the present invention, the amplifying channels 34 and 35 respectively yield pulses of different amplitudes, all of which are delivered through the conductors 5| and 52 to primary coils 53 and 54, respectively, of a transformer 55 having a common secondary coil 1 03 in series connection with the cable conductor 25, so that all of the amplified output pulses from the detectors are transmitted to the cable 24 and by it carried to the surface. Circuits which may be employed in the amplifier channels 34 and 35 are more fully illustrated and described in connection with Fig. 3 of the drawings.

The amplified pulses from the logging instrument, which-are taken iron the cable at the surface in the usual manner at the: cable spooling mechanism, indicated generally at 26, are first directed to a filter 56- which separates the pulses from the supply current, and then pass to a selector 51 in which circuits are provided whereby the total of the pulses derived from both detectors may be transmitted to the amplifier 58 or, if desired, the individual pulses from either one or both of the detectors may be transmitted separately to the amplifier. The circuits in the selector 51 are-shown in Fig. 4.

The pulses transmitted to the amplifier 58 are there further amplified and transmitted to the recorder 59, where they are correlated with indications of the depth of the instrument, transmitted from the reel 26, these indications being transmitted in known manner.

In Fig. 3, the circuits for the detectors 28 and 30 are shown. In this drawing, the detectors are illustrated schematically by a conventional representation of their electrodes. The circuits for the heating filaments for the various electronic tubes are not shown, since such circuits are wellknown and their showing would unnecessarily complicate the drawings. Where ground connections are conventionally illustrated, it is to be understood that these are connections to the base point of common potenial which will in general be the housing of the instrument.

In the circuits illustrated in Fig. 2, the pulses derived from each of the detectors. resulting from radioactivity efiects, are amplified and equalized through similar individual circuits, of which one will be described in detail and corresponding parts in the other will be indicated by the same numerals, carrying a prime The amplified pulses from one of the detectors, preferably the longer one, 30, are reduced in amplitude before being impressed upon the transmitting cable 24.

A high direct current potential, somewhat above that required for operating the detectors,

is drawn from the power pack through the conductors 32 and 33 to be impressed upon the anodes of the detectors 28 and 30, respectively. This high potential is impressed upon the detector 28 through the resistor 60 and conductors GI and 82. It is also impressed upon the plate circuit of a high vacuum electronic tube 63, which is interconnected with a second high vacuum electronic tube 84 in multivibrator relationship, as disclosed in the application of Donald G. C. Hare, Serial No. 581,647, filed March 8, 1945.

The plate 65 of tube 53 thus has impressed upon it through the conductor 66 the same high voltage as is impressed upon the anode of the detector. The plate of tube 63 and the anode of the detector are coupled through the condenser as with the control grid 68 of the second tube 64 of the multivibrator. The condenser 69 blocks -1e transmission of direct current voltage to the control grid 68 of tube 64, but permits the transmission of pulses resulting from the action of the detector 28. The plate 18 of tube 64 is coupled through condenser 14 with the control grid 13 of tube 63. Condenser l4 permits the transmission of pulses from the plate circuit of tube 64 to the control grid. of the tube 63 while blocking direct current fiow.

The cathode 14 of tube 63 is connected to a suitable point of low potential. For example, it may be connected through the conductor 15 with a conductor 16 carrying a suitable low potential, somewhat above ground potential. The control grid 13 of tube 63 is biased byconnecting it through resistor 11 to the housing or ground. The cathode 16 of tube 84 is connected to ground, and the control grid 68 of tube 64 is kept at zero bias by the connection through resistor 18- to ground. The necessary voltage for the plate 10 of tube Be is supplied through resistors and 80 from a source of suitable direct current voltage in the power pack, through the conductor 82.

As illustrated in Fig. 3, the tubes 63 and 64 may be pentodes, for example, such as 7C7 tubes. The voltage for their respective screen grids 83 and 84 is derived from a suitable source of the necessary direct current voltage in the power pack, through conductor 85. The suppressor grids 83 and 89 are each connected with their respective cathodes.

As will be apparent, the plate operating voltage for the tube 63 will be that directly applied to the anode of the detector 28. In a particular case employing one of the types of high efiiciency detectors previously referred to, this voltage may be in the order of 1000 volts. A low direct current voltage, derived from the conductor 16, and which may be in the order of, say, 22-volts is impressed upon the cathode 14 of tube 86, in order that a substantial bias may be placed upon the control grid of this tube 63. A somewhat lower plate operating voltage is placed upon the plate Hi, this suitably being in the order of 200 volts. The discharges resulting from the action of the detector 28 produce negative pulses which are impressed upon the control grid 88. of tube 54. These in turn produce positive pulses in the plate circuit of tube 64 which are transmitted to the control grid 13 of tube 83. This results ina reduction in potential at its plate 65 and an enhancement of the effect of the pulses produced by the detector, and the production at the output of the tube G l, of positive pulses of substantially equalized amplitude, as more fully described in the aforesaid application of Hare, Serial No. 581,647. At the same time, as pointed out in that application, through the modified multivibrator circuits employed, rapid quenching of the detector is secured and it is made sensitive to very small and very rapid pulses. As pointed out above, the same circuits are employed and the same results are secured in connection with the operation of detector 33 as those which have been described in connection with detector 28.

The amplified and equalized pulses which are secured from each of the detectors are then further amplified through the amplifying tubes 9i! and 86', which are power amplifiers, suitably pentodes such as SAG? tubes and preferably operated at about cut-ofi, yielding substantially equalized output pulses. The positive pulses ooming from the plate of the tube 84 are impressed upon the control grid 91 of amplifier tube 98 through condenser 93. The cathode Sit of tube 98 is connected to the same source of low potential (conductor ?6) as is the cathode of tube 63, this connection being efiected through the conductor 85. The control grid 9| of tube 96 is biased by being connected through resistor 86 to ground.

A similar power amplifying tube 98 is provided in connection with the circuits for the detector 28, and corresponding parts of and connections for this amplifier are indicated by the same numbers applied to those of amplifier 9% with the addition of a prime. However, means are provided for attenuating or reducing the amplitude of the pulses delivered byone of thedeteetors,

CROSS REFERENCE '7 these being shown in Fig. 3 in connection with the circuits for detector 30, which represents the longer detector in the instrument.

Referring to the circuit for the detector 28, the amplified pulses from the amplifying tube 90 are transmitted from the plate 91 of this tube through the conductor 5| to the primary coil 53 of the transformer 55, the opposite end of this coil being connected with the conductor 82, from which the necessary voltage for operation of the plate circuit of tube 90 is thus supplied.

The tube 90, which amplifies the pulses from the detector 30, delivers amplified pulses from its plate 81' through conductor 52 to a second primary coil 54 on transformer 55, the opposite end of this coil being connected directly with the conductor 82 supplying the necessary plate operating voltage for the tube 90. A resistor I is series-connected in the line 52 and a second resistor IOI is connected in shunt or parallel with the primary coil 54 between the conductor 52 and the conductor 82 through the conductor I02. It is thus apparent that the resistors IOI and I00 form an attenuator, which serve to attenuate or reduce the amplitude of the pulses impressed upon the primary coil 54 of transformer 55.

The transformer 55 has a single secondary coil I03, which is connected in series with the cable conductor 25, a choke coil I04 being shunted across this secondary to by-pass the power current, which is an alternating current of low frequency, and prevents it from reacting back upon the pulse transmitting system. As is readily apparent, the pulses transmitted through the cable conductor will thus be both the larger amplitude pulses resulting from the discharges in the detector 28 and the lower amplitude pulses resulting from the discharges in the detector 30.

As an illustrative embodiment of the circuits illustrated in Fig. 3, in which the voltage impressed across the electrodes of detectors 2B and 30 is in the order of 1000 to 1100 volts, the voltages impressed upon conductors 82, 85 and I6 may be respectively 200 volts, 45 volts and 22 volts. The constants of the multivibrator circuits comprising the tubes 53 and 64 and the tubes 63 and 64 respectively may be as set forth in the aforesaid application .of Hare, Serial No. 581,647. The impedances of primary coils 53 and 54 may be in the order of 500 ohms each and of secondary coil I03 in the order of 50 ohms. The resistances of resistors I00 and IOI may be selected to secure the desired radiation in amplitude of the pulses; for example, they may be 2000 and 5000 ohms respectively.

As has been hereinbefore stated, when these pulses reach the surface, after passing through a filter which eliminates the alternating current supply from the pulse selecting and amplitying equipment, they pass into a selector, by which a selection may be made of the pulses to be further amplified and recorded and either the pulses or lower amplitude or those of higher amplitude or the total of both may be selected for recording, suitably after further amplification. The selector circuits are more fully illustrated in Fig. 4

As shown in this figure, the cable conductor 25 is coupled through condenser I05 with the filter 56, of which details are not shown, as these are well-known in the art. This filter serves to suppress the frequency of the current from the alternating current source 44, which may suitably be.60 cycles. From the filter 56 pulses are impressed upon a conductor I01, from which,

through a hand operated switch I 08, they may be impressed either upon conductor I09 or conductor H0. The switch is shown in position for transmitting the pulses to the conductor I09, in which case either the total of the pulses transmitted by the cable or only the pulses of larger amplitude may be recorded, as desired. The selection between these alternatives is made as hereinafter indicated.

The conductor I09 is connected through a condenser III and conductor II2 with the control grid II3 of an amplifying tube II4, suitably a high-mu triode or one unit of a duplex triode such as a '7F7 tube. A suitable operating voltage is impressed upon the plate II5 of the tube I I4 through the resistor H6; and the cathode I I8 of tube I I5 is grounded.

In order that a selection of pulses may be made between the total pulses and the pulses of larger amplitude, provision is made for varying the bias voltage placed on the control grid of the tube I I4. By use of a lesser bias on this tube, all of the pulses, both of larger and smaller amplitude, delivered from the cable 25, can be caused to produce pulses in the plate circuit of tube II4. By increasing the negative bias on the control gridII3 of tube II4, the effect of pulses of smaller amplitude is eliminated and only the pulses of larger amplitude produce pulses in the plate circuit of tube I I 4. This change in bias on the control grid of tube II4 may be effected in any desired manner, one circuit suitable for this purpose being illustrated. In this, the bias on the control grid II3 of tube H4 is provided by the battery I20 and poteniometer I2I from which the desired negative voltage may be taken by means of switch I22, to be impressed on the control grid II3 of tube II 4. According to the negative bias applied, the tube responds to all of the pulses delivered from the cable or only to those of larger amplitude. Since, in the circuits illustrated in Fig. 2, the pulses of larger amplitude are those produced by the shorter detector 30, it is apparent that by proper selection of the bias voltage on the tube II4, the latter can be made to respond only to the pulses resulting from the action of the shorter detector.

The pulses resulting from the action of the tube II4 are delivered to the amplifier system 58 and the recorder 59, in which they may be amplified and recorded in usual manner.

In some cases, it may be desired to record the pulses of smaller magnitude alone or to record them independently of the pulses of larger magnitude delivered from the cable 25. Suitable circuits for accomplishing this are also shown in Fig. 4. In such case, the control switch is thrown to contact the terminal of the conductor H0. The pulses are then impressed on the conductor H0 and are transmitted to the control grids I32 and I33 of two high vacuum electronic tubes I34 and I35, shown as triodes, suitably triode units of a tube of the 7F7 type. The plates I36 and I3! of the tubes I34 and I35 respectively have impressed upon them the necessary plate operating voltage through conductors I38 and I39, both of which are connected at a common point I40 with a conductor I4I which is in turn connected through a load resistor I 42 and conductor I43 with a suitable source of plate operating voltage.

Tube I34 has its control grid I32 biased, for example, by means of biasing battery I44 and biasing resistor I 45. Tube I 35 has its control grid I33 biased by means of the biasing battery I46 and resistor I41. The negative bias voltage EXAMINER on the control'grid; of the tube I: is greaterthan that onthe tube I80, being adjusted so that the pulses: of smaller amplitude do not cause plate cmrent to flowin this tube, althoughthe pulses of larger amplitude cause plate current to flow therethrough. The bias onthe control grid of the tube Ir is.-so adjusted that plate current is caused to flow through this tube by either the pulses of smaller amplitude or those of larger amplitude.

The primary winding I 428 of a.- transformer is connected in series. in the plate circuit of the tube [34' and itssecondary winding I49 is coupled through a. condenser Hi0 to the amplifier 58.

Wherra small! pulse is delivered from the transcable conductor to. the conductor H0, it causes current to flow in tube I34, producing a pulse in the plate circuit of this tube, which is transmitted through the transformer coils I48 andwe tothe conductor i507 and thereby to the amplifier 58. When a pulse of larger amplitude is delivered tothe conductor H0, plate current is caused to flow through the tube ['35 as well as the tube I534. The tube N5 is selected so that the plate current which flows through it under these circumstances will cause a substantial drop in voltage at the common point of connection I40 of the plate circuits of bothtubes with the plate resistor Mt. As a result, the plate voltage on the tube I-34' will be lowered to such an extent that there is no substantial amplification of the pulse delivered to the control grid 132i of this tube. In order to insure that this effect will take place, it is preferred that there be a small time delay between the operation of tube I35 and I34, which may be effected by selecting the constants of the coupling condenser and grid resistor in the control grid circuit of tube I 34 so that their product is somewhat greater than the product for the corresponding constants of tube r35. In this way only the small pulses taken from the cable conductor 25 will cause the production of a pulse of appreciable size in the conductor I and hence in the amplifier 58 and recorder 59.

It is thus apparent, by a suitable selection of the positions of switch W8 and tap I22 it is possible to amplify andrecord' the pulses from either detector in the well-logging instrument or the total of the pulses from both. Although in the specific illustration shown the smaller pulses are derived from the longer detector and the larger pulses from the smaller detector, the reverse arrangementmay be used. It is also readily apparent that the selector system of the present invention may be used in connection with instruments containing a plurality of detectors of the same length. Also, by suitable connections to separate amplifying and recording systems, by the circuits of the present invention, the Pulses from the detectors may be individually amplified and recorded or they may be separately recorded at the same time, and correlated with suitable indications or recordings of the depth of the instrument.

Additional detectors may be present in the detector and by reversing the polarity of their pulses when applied to the transmission cable, they may be separated by suitable equipment at the surface. Thus, up to four detectors may be carried by the well-logging instrument, each pair giving pulses of the same polarity but of difierent amplitudes, and the resulting pulses of each polarity may then be handled in accordance with the present invention.

The invention has been described in connection with the use of radiation detectors for the detection of natural gamma radiation. By providing a suitable source of penetrating radiation in or associated with the instrument containing the detectors and properly shielded therefrom, the radiations acting upon the are modified or influenced thereby and the induced radiations are detected and recorded. The source of. radiation thus used may be any suitable source of penetrating radiations, such as gamma rays or neutrons. By suitable modification the detectors may be caused to respondto other radiation than to gamma rays for example, by coain'ng the interior thereof with a boron or lithium compound or including boron trifiuoride in the gas contained therein they may be caused to respond to slow neutrons, by filling the counter with hydrogen or by coating the cathodes with a. hydrogen-containing material such as paraflin waxor with indium, they may be caused to respond to fast neutrons.

Although the invention has been described in connection with the details of specific embodiments thereof, it is not intended to thereby him; the invention except in so far as such details are included in the accompanying claims.

What I claim is:

1. A method of logging a bore-hole which comprises moving through said hole a pair of vertically elongated radiation detectors of different lengths, one of said detectors being substantially as long as the vertical dimension of the thinnest stratum to be logged, recording the output of said one detector while the detectors are moving through the hole in the vicinity of a thin stratum, and recording the total output of both detectors while the detectors are moving'through other portions of the hole.

2. A method of logging a bore-hole which comprises moving throughsaid hole a pair of vertically elongated radiation detectors of difierent lengths of the counter type, the shorter of said detectors being approximately as long as the vertical dimension of the thinnest stratum to be logged and the longer detector being substantially three times as long as the shorter detector, recording the output of the shorter detector while said housing is moving through the hole in the vicinity of a thin stratum, and recording the total output of both detectors while the housing is moving through other portions of the hole.

3. A method of logging a bore-hole which comprises moving through said hole an instrument containing a pair of vertically elongated radiation detectors of diiierent lengths, the shorter of said detectors being substantially as long as the vertical dimension of the thinnest stratum to be logged, recording the output of the shorter detector while said instrument is moving through the hole in the vicinity of a thin stratum at a comparatively slow speed; and recording the total output of both detectors while the instrument is moving through other portions of the hole at a higher speed whereby the thin and the thick strata will be logged with substantially the same accuracy.

4. The method of making an accurate radiological log of a bore-hole which comprises passing through said hole a device including two sensitive radiation detectors of the counter type, said detectors being of substantially difierent lengths, moving said device at a speed of approximately 1000 feet per hour while formations varying from one to five feet in thickness are being traversed while recording the response of the shorter of said detectors and moving said device at a speed of at least 4000 feet per hour while traversing formations greater than five feet in thickness while recording the total of the responses of both detectors in correlation with the depth of the instrument.

5. An apparatus for making a radiological log of the formations traversed by a bore-hole, which comprises a housing suspended in said hole by means of a single conductor cable, a pair of elongated radiation detectors of substantially different lengths disposed in said housing, means in said housing adapted to connect either of said detectors to said cable, the shorter of said detectors being substantially as long as the vertical dimension of the thinnest stratum to be logged, filtering and recording means at the surface connected to said cable for separately recording the output signals from one of said detectors and the total of the output signals from both of said detectors, means at the surface connected to said cable for supplying electrical energy to said detectors, and means for lowering and raising said housing in said hole so that said detectors will be exposed to radiation emitted from said formations.

6. A method of logging bore-holes which comprises passing along the bore-hole a plurality of radiation detectors of the pulse-producing type, separately amplifying and equalizing the output pulses of said detectors, reducing the amplitude of the amplified pulses from one of said detectors. combining said amplified pulses in a single conductor, conducting said pulses to the surface, and selectively recording values, one of which is a function of the pulses from one of said detectors and another of which is a function of the total of said pulses.

'7. A method of logging bore-holes which comprises passing along the bore-hole a plurality of radiation detectors of the pulse-producing type, separately amplifying and equalizing the output pulses of said detectors, reducing the amplitude of the amplified pulses from one of said detectors, combining said amplified pulses in a single conductor, conducting said pulses to the surface, and selectively recording values, one of which is a function of the amplified pulses of greater amplitude from one of said detectors and another of which is a function of the total of said pulses.

8. A method of logging bore-holes which comprises passing along the bore-hole a plurality of radiation detectors of the pulse-producing type, amplifying the output pulses of each detector to a constant magnitude which is different for the pulses of each detector, combining said amplified pulses in a single conductor, conducting said pulses to the surface, and selectively recording values, one of which is a function of the pulses from one of said detectors and another of which is a function of the total of said pulses.

9. A method of logging bore-holes which comprises passing along the bore-hole a plurality of radiation detectors of the pulse-producing type, amplifying the output pulses of each detector to a constant amplitude which is different for the pulses of each detector, combining said amplified pulses in a single conductor, conducting said pulses to the surface, and selectively recording values, one of which is a function of the pulses of greater amplitude from one of said detectors and another of which is a function of the total of said pulses.

10. A method of logging bore-holes which comprises passing along the bore-hole a plurality of radiation detectors of the pulse-producing type, amplifying the output pulses of each detector to a constant amplitude which is different for the pulses of each detector, combining said amplified pulses in a single conductor, conducting said pulses to the surface, and selectively recording values, one of which is a function of the pulses of lesser amplitude from one of said detectors and another of which is a function of the total of said pulses.

11. An apparatus for logging bore-holes which comprises an instrument housing adapted to pass through the bore-hole, a plurality of radiation detectors of the pulse-producing type, means in said housing for amplifying the output of each detector to produce amplified and equalized pulses, means in said housing for attenuating the amplified pulses derived from one of said detectors, means for combining the output of said detectors and transmitting them to the surface, recording means at the surface, and means for delivering the pulses to the recording means, said means including means for selectively eliminating from the pulses delivered to the recording means the pulses from one of said detectors by the relative amplitudes thereof, whereby either the total of the pulses or only the pulses of greater amplitude are recorded.

12. An apparatus for logging bore-holes which comprises an instrument housing adapted to pass through the bore-hole, a plurality of radiation detectors of the pulse-producing type, means in said housing for amplifying the output of each detector to produce amplified and equalized pulses, means in said housing for attenuating the amplified pulses derived from one of said detectors, means for combining the output of said detectors and transmitting them to the surface, recording means at the surface, and means for delivering the pulses to the recording means, said means including means for selectively eliminating from the pulses delivered to the recording means the pulses of reduced amplitude, whereby either the total of the pulses or only the pulses of greater amplitude are recorded.

13. An apparatus for logging bore-holes which comprises an instrument housing adapted to pass through the bore-hole, a plurality of radiation detectors of the pulse-producing type means in said housing for amplifying the output of each detector to produce amplified and equalized pulses, means in said housing for attenuating the amplified pulses derived from one of said detectors, means combining the output of said detectors and transmitting them to the surface, recording means at the surface, means for delivering the pulses to the recording means, said means including means for selectively eliminating from the pulses delivered to the recording means the pulses of greater amplitude, whereby either the total of the pulses or only the pulses of lesser amplitude are recorded.

14. An apparatus for logging bore-holes which comprises an instrument housing adapted to pass through the bore-hole, a plurality of radiation detectors of the pulse-producing type means in said housing for amplifying the output of each detector to produce amplified and equalized pulses, means in said housing for reducing the amplitude of the amplified pulses derived from one of said detectors relative to those derived from the other, means for combining the amplified pulses from said detectors and transmitting them to the surface, recording means at the surface, means for delivering the pulses to the recording means, said means including an electronic tube having a biased control grid, and means for varying the bias voltage on the control grid of said tube, thereby selectively eliminating from the pulses delivered to the recording means the pulses of reduced amplitude, whereby either the total of the pulses or only the pulses of greater amplitude are recorded.

15. In apparatus for selecting pulses of lesser amplitude from unidirectional pulses of varying amplitudes, a pair of high vacuum electronic tubes, each having a control grid and means for impressing a negative bias voltage thereupon, the negative bias voltage impressed on the control grid of one of said tubes being greater than that on the control grid of the other, connections from the plates of said tubes to a common plate load resistor from which plate current is provided for both of said tubes, means for impressing the pulses of varying magnitude upon the control grids of both of said tubes, whereby the tube having the control grid with greater bias is caused to pass current by the pulses of greater amplitude and to thereby reduce the plate operating 14 voltage on the other tube which has its bias adjusted to a point where it becomes conducting by the smaller pulses, coupling means for delaying the pulses delivered to the tube with smaller bias compared to the pulses delivered to the tube of larger bias so that the tube with smaller bias produces substantial pulses only when its control grid receives pulses of smaller amplitude, and means for transmitting such pulses from the plate circuit of said other tube.

GERHARD HERZOG.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,298,794 Howell Oct. 13, 1942 2,369,550 Frosch Feb. 13, 1945 2,370,162 Hare Feb. 27, 1945 2,378,408 Herzog June 19, 1945 2,408,063 Grieg Sept. 24, 1946 2,419,548 Grieg Apr. 29, 1947 

